Vibration damped sandwich systems



Dec. 15, 14970 Filed NOV. 18, 1968 H. OBERST ETAI- vIBRATioN DAMPED SANDWICH SYSTEMS 2' Sheets-Sheet 1 ATTORNEYS DEC. 1 5, 1970 H, @EERST ETAL 3,547,756

VIBRATIGN DAMPED SANDWICH SYSTEMS Filed NOV. 18, 1968 2 Sheets-Sheet B Illlllllllllllll FIG. 2G

lllllllllllllllll INVENTGF'S BY 1g-1%M` www AT TORNEYZ United States Patent Otlice 3,547,756 Patented Dec. 15, 1970 3,547,756 VIBRATION DAMPED SANDWICH SYSTEMS Hermann Oberst, Hofheim, Taunus, Joachim Ebigt and Gnther Duve, Frankfurt am Main, and Alfred Schornmer, Bad Soden, Taunus, Germany, assignors to Farb- Werke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany Filed Nov. 18, 1968, Ser. No. 776,505 Claims priority, application Germany, Dec. 5, 1967, 1,700,124 Int. Cl. B32b 15/08; C08f 19/02; E04b 1/99 U.S. Cl. 161-165 5 Claims ABSTRACT OF THE DISCLOSURE A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising a graft polymer of styrene or optionally a mixture of styrene with a copolymerizable carboxylic acid, for example acrylic and/ or methacrylic acid, on copolymers of 30 to 50% by weight of vinyl acetate and 70 to 50% by weight of ethylene.

The present invention relates to vibration damped sandwich systems having interlayers made of a graft polymer of styrene or optionally a mixture of styrene with a copolymerizable carboxylic acid, especially acrylic acid and/ or methacrylic acid, on copolymers of vinyl acetate and ethylene.

It is known from South African specification No. 5,269 that highly valuable vibration damping materials of a broad temperature band suitable for damping bending vibrations of metal sheet constructions can be prepared by copolymerizing monomers whose homopolymers differ in their second order transition temperature by at least C. The above specification also reports that as vibration damping materials having a broad ternperature band there can be used, among others, predominantly amorphous copolymers of ethylene and vinyl esters of fatty acids with 2 or 3 carbon atoms, for example vinyl acetate/ethylene copolymers. The ethylene/ vinyl acetate copolymers used up to now are, however, not satisfactory in all respects.

It has now been found that graft polymers of styrene or optionally a mixture of styrene with a small amount of a copolymerizable carboxylic acid, especially acrylic and/or methacrylic acid, on copolymers of vinyl acetate and ethylene have outstanding damping properties and a very broad temperature range of damping and are, therefore, especially suitable for the vibration damping of sandwich systems of hard plates, in particular metal sheets. Suitable copolymers of vinyl acetate and ethylene are preferably those containing about 30 to 50% by weight of vinyl acetate, especially about 40% by weight of vinyl acetate and, consequently, about 70 to 50% by weight of ethylene. From the economical point of view they have the advantage of being rather cheap.

The graft polymers of styrene or optionally a mixture of styrene with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) on copolymers of vinyl acetate and ethylene are obtained by dissolving the specitied copolymer in monomeric styrene or optionally a mixture of monomeric styrene and copolymerizable carboxylic acid (acrylic and/ or methacrylic acid) containing a catalyst and performing a free-radical initiated polymerization at a temperature in the range of from about 60 to about 180 C. Especially good results are obtained with graft polymers of 55 to 90% by weight of styrene or a mixture of styrene with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) on 45 to 10% by weight of a copolymer as described above, for example a graft polymer of 60% by Weight of styrene, or 60% by weight of a mixture of 90% by weight of styrene and 10% by weight of acrylic and/or methacrylic acid, on 40% by weight of a copolymer as specified above. As polymerization initiator there may be used tertiary b-utylhydroperoxide in the usual concentrations. With graft polymers of this type, the vibration damping effect of which critically depends on the weight proportion of the monomers, very broad damping curves can be obtained having very high maximum damping values.

The present invention thus provides sandwich systems of hard plates, in particular metal sheets, having vibration damping self-adherent interlayers consisting of graft polymers of styrene or optionally a mixture of styrene with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) on copolymers. of vinyl acetate and ethylene, for which interlayers there are used graft polymers of 55 to 90% by weight of styrene or a mixture of styrene with 0.1 to 10% by weight, calculated on the styrene, of a copolymerizable carboxylic acid (acrylic and/or methacrylic acid), on 45 to 10% by weight of a copolymer of 30 to 50% by weight of vinyl acetate and to 50% by weight of ethylene.

FIGS. la and 1b of the accompanying drawings are plots against temperature of the loss factor dwmb to illustrate the superior eiiiciency of the novel systems. The curve in FIG. 1a shows the loss factor dwmb of a metal sheet arrangement of the invention as a function of temperature. For comparison, one of the most effective vibration damping materials known for metal sheet arrangements was used, namely a copolymer of 63% by weight of vinyl acetate and 37% by weight of dibutyl maleate containing a plasticizer 15% by weight of 2-ethylhexyl phthalate and 15% by weight of tricresyl phosphate, calculated on the mixture (curve of FIG. lb). The copolymer of curve 1b was a thermoplastic adhesive especially suitable for producing vibration damped metal sheet sandwich systems comprising two outer metal sheets and a self-adherent thermoplast as damping interlayer. Systems of this type provide a damping effect which is extremely high in its maximum and cannot be exceeded for physical reasons (cf. H. Oberst and A. Schommer, Kunststoffe, volume 55, page 6134 (1965), especially FIG. 9). In a symmetrical arrangement comprising two metal sheets, each having a thickness of 0.5 millimeter, and an interlayer 0.3 millimeter thick, the loss factor dcomb of the combined system, measured in the bending wave method (cf. for example H. Oberst, L. Biohn and F. Linhardt, Kunststolfe, volume 51, page 495 (1961) almost reaches the value dcomb of 1. The known metal sheet damping by one-side damping coatings which are applied by spraying, trowelling or bonding in the form of layers of socalled vibration damping materials show loss factors generally of less than dcom5=0n2 with technically reasonable thicknesses or ratios of coating mass to plate mass of the combined system. With metal sheet sandwich systems which gain growing importance in recent times, it is possible to obtain damping values that are increased by a multiple, as shown by the present example, when the interlayer material has the appropriate composition and thickness.

The temperature band width of the damping of the metal sheet sandwich system does not only depend on the viscoelastic properties of the interlayer and the steel sheets but also to a considerable extent on the geometry of the arrangement, i.e. on the ratio of the layer thicknesses (cf. loc. cit. (1965), FIGS. 8 to 10). With metal sheet sandwich systems the band width is advantageously defined as the range of the temperature interval Within which the value dcomb=-05 is exceeded. The damping of metal sheets which are not damped by additional vibration damping means is metal sheet construction of Various types corresponding to values dcomb 0.01. The reference value dcomb of 0.05 thus means a considerable increase in the damping effect (by about db (decibel)) as compared to the nil damping dcmb=0.01.

In the curve shown in FIG. 1b the reference value dcomb of 0.05 is exceeded in the mainly interesting frequency range of from 100 to 1,000 c.p.s. (Hz.) at temperatures ranging from about 0 to 50 C. The temperature band width thus corresponds to about 50 C. Sandwich systems of this type are suitable for many technical fields of application. By modifying the content of plasticizer, it is possible to shift the temperature band of a high damping effect to higher temperatures and thus to adapt the material to special technical uses, for example in machine units operating at elevated temperatures. This example of a metal sheet sandwich system comprising a self-adherent interlayer having optimum properties of a vibration damping material with a broad temperature band prepared by copolymerizing appropriate monomeric compounds has hitherto not been surpassed by other arrangements of similar kind and may be taken as standard for judging the acoustic eiiciency of the system according to the invention.

FIGS. 1a and 1b show the temperature curves of the loss factor dccmb of metal sheet sandwich systems comprising steel sheets of a thickness of 0.5 millimeter each and damping interlayers of a thickness of about 0.8 and 0.3 millimeter, respectively (FIG. 1b) for a frequency of 100 c.p.s. and 1,000 c.p.s.

The curves were measured with sandwich systems the interlayers of which consisted of (la) A graft polymer of 60% by Weight of a mixture of 90% by weight of styrene and 10% by weight of acrylic acid on 40% by Weight of a copolymer of 40% by Weight of vinyl acetate and 60% by weight of ethylene (according to the invention) (lb) A copolymer of 63% by weight of vinyl acetate and 37% of dibutyl maleate containing as plasticizer 15% by weight of 2-ethylhexyl phthalate and 15 by weight of tricresyl phosphate, calculated on the mixture.

The arrangement la, whose monomer proportion lies 1n the optimum range has a very broad temperature band Width with relatively high maximum damping values. With 1 the arrangement 1a the center of damping is at about 110 C. at 100 c.p.s. and at about 30 C. for 1,000 c.p.s. with maximum damping values of about 0.4 and 0.5 respectively. The temperature band width is about 175 C. for 100 c.p.s. and about `'160 C. for 1,000 c.p.s. In arrangement la, the slow decrease of the damping towards high temperatures and with 10W frequencies of about 100 c.p.s. also towards low temperatures is especially favorable. The excellent vibration damping properties are maintained with reference to dcomb=0-05 within a temperature range of about 0 C. to .about 165 C. As compared with the standard system 1b, arrangement la has an appreciably broader temperature band width and beter vibration damping properties at a temperature above 45 C., so that arrangements of this type are suitable for quite a number of applications at elevated temperature (for example machine units and appliances operated at elevated temperature). Owing to its content of 10% by weight of acrylic acid the graft polymer used in arrangement la can be cross-linked by reaction with a bifunctional or trifunctional compound (for example a compound containing a plurality of epoxide, isocyanate or similar groups), whereby the softening range and therewith the range of high damping can be noticeably shifted towards higher temperatures.

A special advantage of the vibration damping material of the present invention resides in the fact that it may be applied continuously during the mass production of the metal sheet sandwich systems.

For this purpose it can be applied (l) in the form of the nished graft polymer and (2) as a solution of the copolymer in the styrene or styrene and acrylic and/or methacrylic acid mixture to be grafted on, which contains the polymerization initiator. In the latter case the graft polymer is produced by subjecting the sandwich system comprising the solution as interlayer to a thermal treatment at a temperature in the range of from about 60 to about 180 C. It is a thermoplastic adhesive which may be applied to the metal sheets by trowelling, brushing or pouring at elevated temperature. The sandwich system may then be advantageously cooled under pressure between rollers. Except for degreasing, the metal sheets do not require a preliminary treatment and further adhesive. Owing to the content of the copolymer of acrylic acid and/or methacrylic acid, degreasing may even be dispensed with. The adhesion is very good.

The vibration damping material of the present invention has a good resistance to flow. The metal sheet sandwich system may, within broad limits, be processed as normal metal sheets, that is they may be creased, bent, shaped, welded and riveted. In this manner laminated systems are obtained having a damping height and a temperature range of damping which makes them well suitable for many applications, especially at normal and elevated temperatures.

Minor amounts of fillers, for example for improving the electric conductivity (improvement of resistance welding) may be incorporated in the vibration damping materials. In order not to affect the damping eifect adversely it is advantageous to use less than 1% by weight, preferably less than 0.5% by weight of filler, calculated on the polymer. Suitable llers are, for example, heavy spar, silicic acid, graphite and soot.

The metal sheet sandwich system suitably has a total thickness in the range of from 1 to 6 millimeters. The interlayers may have a thickness of 0,1 to 1 millimeter, preferably 0.2 to 0.5 millimeter. A maximum damping effect is obtained with symmetrical laminated systems. With an equal weight, asymmetrical laminated systems have, however, a higher stiffness in ilexure and strength. Asymmetrical laminated systems are, therefore, preferred for those applications which require a high strength with respect to the Weight. The ratio of the thicknesses of the outer plates or metal sheets is preferably within the range of from 1:1 to 1:4.

FIG. 2 of the accompanying drawings shows sandwich systems with symmetrical arrangement a and asymmetrical arrangement b in which the interlayer 2 is interposed between the two outer plates or metal sheets 1.

What is claimed is:

1. A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising a graft polymer of 55 to 90% by weight of styrene or a mixture of styrene with 0.1 to 10% by weight of a copolymerizable carboxylic acid, calculated on the styrene, on 45 to 10% by weight of a copolymer of 30 to 50% by weight of vinyl acetate and 70 to 5 0% by weight of ethylene.

2. The sandwich system of claim 1, wherein the carboxylic acid of the graft polymer is selected from the group of acrylic acid, methacrylic acid and mixtures of these two acids.

3. A vibration damped sandwich system as claimed in claim 1, wherein the hard plates are metal sheets.

5 y 6 4. A `vibration damped sandwich system as claimed 3,314,904 4/1967 Burkus. in claim 1, wherein the graft polymer of the interlayer 3,355,516 11/ 1967 Hardt et al. contains up to 1% by Weight of a filler, calculated on 3,399,103 8/1968 Salyer et al. 3,414,638 12/1968 Hardt et a1.

the graft polymer.

5. A vibration damped sandwich `system as claimed in claim 1, wherein the ratio of the thicknesses of the plates lies in the range 0f from 1:1 to 1:4. JOHN T- GOOLKA'SIAN, Pflmall'y EXamlUef G. MO II References Cited W XON Ass1stant Examiner UNITED STATES PATENTS 10 U.S. C1. X.R..

3,218,373 11/1965 Salyer. 3,271,188 9/1966 Albert et al.

3,433,724 3/ 1969 Gif-sur-Yvette et al. 

